The objective of this proposal is to develop and validate a practical clinical laboratory test for objectively measuring patient adherence to diets of controlled content of polyunsaturated fat as used for the clinical treatment and prevention of hyperlipidemia and atherosclerotic heart disease. Studies published by Holman and colleagues over 1961-1970, using analysis by alkali isomerization, demonstrated a linear relationship between the logarithm of the percent of total diet calories as linoleic acid and the proportions of polyunsaturated forms in the fatty acids of the complex lipids of serum and some tissues of several species and the serum of infants and young men. From this he proposed to use the measure of serum polyunsaturated fatty acids as a measure of diet intake of linoleic acid. The present proposal has four major aims: 1) to validate and define more precisely the relationship of linoleic acid in the diet to the polyunsaturated fatty acids in serum cholesteryl esters and triglycerides in older men using the more specific and modern analytical techniques of gas liquid chromatography of fatty acids and feeding of diets containing linoleic acid at 5 to 12 percent of calories on the metabolic research ward; 2) to determine the optimum conditions of sampling of serum, the lipid species and diet duration for using the relationship, and to design the laboratory test of adherence; 3) to determine the effect, on this relationship and the test, of reduction in body weight if one to two pounds a week as is commonly combined with the diet in clinical practice; 4) to determine the effects of the proportion of fat in the diet by comparing relationships of serum and diet linoleic acid when feeding diets with 32 and 40 percent of calories from fat, levels cmmonly encountered in clinical practice.